There are three students X, Y, and Z. One of them is Alpha who always tells truth. Beta always lies and Gamma can tell truth as well as lie. X declares that he is not Beta. Y says he is Beta. Z says he is not
Beta. Who is Gamma?....(GIVE REASON)

The gamma is Y. Y says he is the beta. If he is the alpha then his statement becomes false and alpha can`t lie. If he is beta then his statement becomes true n beta can`t tell the truth. Hence Y is neither alpha nor beta. So he is gamma and is lying.